"We put atmospheric signatures that guarantee someone with a large telescope 20 light years away could detect us," said John Grunsfeld, associate administrator for NASA's Science Mission Directorate. "If there is life out there, intelligent life, they’ll know we’re here."
Grunsfeld, a former astronaut, pointed to changes that life on Earth has created in the environment as one of the telltale signs intelligent life could spot Earthlings from afar.
"'Are we alone?' is the biggest driving question," Grunsfeld said.
NASA's chief scientist Ellen Stofan said in April that she believes the first indications of alien life could come by 2025 -- with more concrete evidence of extraterrestrial life coming in 20 to 30 years.
While Stofan predicts mankind is likely not alone in the universe, she said at a panel discussion about water in the universe she expects alien life won't be "little green men."
"We are talking about little microbes," she said.